Water harvesting device

ABSTRACT

An apparatus extracts water from the air using a single circuit of gas containing a compressor, a cooling element, a heat exchanger and an ion source, where a single stream of air is used to provide water and to pre-cool the inlet air.

TECHNICAL FIELD

This invention provides an apparatus and method for removing water from the atmosphere or other gas. The apparatus and method of the present invention are particularly useful for collecting clean, potable water for consumption or other uses such as cleaning, washing, etc. The apparatus and method are also useful for dehumidification in a particular environment such as a car, airplane, home, or inside of an apparatus, store, or container where dry conditions are desirable. In addition, the apparatus and method of the present invention may be used for accumulating and maintaining or providing a volume of water for uses such as agriculture, domestic ‘grey water’, automobile cooling system, non-sterile washing, cleaning applications, etc. The method of the present invention may furthermore be used for drying air which has been, or will be, used to dry other objects such as clothes, dishes, or human hair. For some applications water can also be removed from air that is purposefully humidified by saline water. The method and apparatus described herein is efficient in that it consumes less energy than previously described apparatus and methods.

BACKGROUND ART

Methods that dehumidify or remove water from the atmosphere are well established in the prior art. The most common method involves bringing the humid gas into contact with a cold surface. This cools the air and lowers its vapour pressure. The excess water condenses onto the surface and the dry air is then circulated back into the room. Devices operating via this technique use a modified refrigeration, evaporative cooling cycle.

Most commercially available dehumidification systems that use this technique first pass air over the evaporator of the system in order to cool the air and condense water onto it. Then the cold, dry air is passed over the condenser to aid the removal of heat from this hot element. More advanced designs are also described, such as that described in U.S. Pat. No. 4,428,205 (Doderer). In this described method, air is again chilled by passing it over the evaporator of the system and water is collected using established techniques. The inlet air is pre-cooled within an air-to-air heat exchanger using the cold, dry air that has passed over the evaporator. The condenser is cooled using a separate flow of ambient air. A related method is described in GB 2064099A (Lawson). It also pre-cools the inlet air within an air-to-air heat exchanger using the cold, dry air that has passed over the evaporator, but in this method, the heat-exchanged exhaust air is passed over the condenser to remove heat from the system. Both of these methods reduce the load on the cooler and can allow it to reach lower temperatures such that more water is extracted for each unit of processed air.

Another common dehumidification method is to absorb the water directly into a desiccant, an example of which is described in US Publication No. 2005/0204914 A1 (Boutall). The desiccant is re-activated, or dried, in a second air flow using heat. A heat exchanger is used to recover some of the heat energy used in evaporating the absorbed moisture.

Still another method involves adiabatically expanding a body of gas, which cools the gas below the saturation point, and water condenses in the form of suspended water droplets. Examples of such method include those described in US Publication No. 2008/0178625 (Thompson et al.), GB2453798 (Strevens) and US Publication No. 2007/0256430 (Prueitt).

Atmospheric Water Generators (AWGs) are also well known in the prior art and are used to collect water from the atmosphere for human consumption. These devices commonly use similar principles to refrigerant cycle dehumidifiers, with additional safeguards to ensure suitability of the water for drinking, washing or irrigation of edible crops. An example of this type of device is the commercially available A2E-28L device (Air2eau Carbon Ltd., UK). This device uses a very simple air flow scheme—the cold, dry exhaust gas is passed directly over the condenser to remove heat from it, but does not attempt to pre-cool the inlet gas.

The prior art devices do not include devices that cool air via adiabatic expansion, in the presence of ions generated by a electric source, and that use a heat exchanger to pre-cool the pre-expanded gas. In the present invention, these functions are achieved using a single stream of air.

SUMMARY OF INVENTION

The method according to the present invention uses adiabatic expansion to cool the air and nucleate water droplets onto purposefully added condensation nuclei, and a heat exchanger to pre-cool the pre-expanded air. In this way a single stream of air is used both to provide water and to pre-cool the pre-expanded air.

In one aspect of the invention there is provided an apparatus for collecting water from air that includes: a compressor for compressing incoming ambient air; a counter-flow heat exchanger having an inbound portion and an outbound portion, the heat exchanger receiving compressed air from the compressor in the inbound portion; an adiabatic expansion cooling device to further cool the compressed air and to condense water therefrom; an ion source for providing condensation nuclei to the adiabatic expansion cooling device; and a water collector for collecting the condensed water from the cooling device. The outbound portion of the counter-flow heat exchanger is configured to receive the cooled dry air from the adiabatic expansion cooling device to pre-cool the compressed air in the inbound portion before the dry air exits the outbound portion.

In one embodiment, the ion source comprises an electrostatic ion generator.

In one embodiment, the apparatus further includes air filter to filter the incoming ambient air.

In one embodiment, the apparatus further includes a humidity sensor.

In one embodiment, the counter-flow heat exchanger includes an air-to-air heat exchanger.

In one aspect of the invention there is provided a method for collecting water from air that includes: flowing compressed air from a compressor to a counter-flow heat exchanger having an inbound portion and an outbound portion, the heat exchanger receiving the compressed air from the compressor in the inbound portion; cooling the compressed air in an adiabatic expansion cooling device; providing condensation nuclei to the adiabatic expansion cooling device from an ion source; condensing water from the cooled compressed air; and collecting the condensed water from the cooling device in a water collector; wherein the outbound portion of the counter-flow heat exchanger is configured to receive the cooled dry air from the adiabatic expansion cooling device to pre-cool the compressed air in the inbound portion before the dry air exits the outbound portion.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF DRAWINGS

In the annexed drawings, like references indicate like parts or features:

FIGS. 1( a) and (b) are schematic diagrams of an embodiment of the device according to this invention using an adiabatic expansion cooling process.

FIG. 2 is a graph of the pressure drop required to initiate droplet formation for adiabatic expansion, with and without ions.

FIG. 3 is a graph of calculated energy cost of an example device operating using inlet air at 27° C. at 40, 50 and 60% RH, with 85% efficient heat exchanger

FIGS. 4( a)-(d) are schematic diagrams illustrating steps in the expansion cycle for devices using adiabatic expansion.

FIG. 5 is a schematic diagram of an embodiment of the device according to this invention using an adiabatic expansion cooling process, which uses a piston to move the cooled air through system.

FIGS. 6( a)-(d) are schematic diagrams illustrating steps in the expansion cycle for devices using adiabatic expansion, which use a piston to move air through the system.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1 compressor     -   2 heat exchanger     -   3 chamber     -   4 valve     -   5 water-droplet collector     -   6 container     -   7 ion containing air     -   8 clean filtered air     -   9 energy required for inlet air 27° C. 40% RH     -   10 energy required for inlet air 27° C. 50% RH     -   11 energy required for inlet air 27° C. 60% RH     -   12 chamber     -   13 fan     -   14 ion source     -   15 valve     -   16 valve     -   17 valve     -   18 valve     -   19 air-to-air heat exchanger     -   20 droplet collector     -   21 humidity sensor     -   22 chamber     -   23 valve     -   24 valve     -   25 valve     -   26 valve     -   27 air-to-air heat exchanger     -   28 piston     -   29 pump     -   30 valve     -   31 water droplet collector

DETAILED DESCRIPTION OF INVENTION

The principle features of this invention can be realised using a variety of methods, examples of which are described below.

In an embodiment, the gas is cooled via adiabatic expansion of compressed gas. Water is collected in the form of suspended droplets, which form around condensation nuclei. These nuclei are provided in the form of ions. The presence of ion nucleation sites allows condensation droplets to form at lower degrees of saturation (higher temperatures) and so improves the efficiency of droplet formation and collection. This embodiment uses the expanded, cold, dry gas to pre-cool the inlet gas. Since this cooling is done during or after compression, it also removes heat from the compression process.

The principle of operation of a water extraction device according to the invention is shown in FIG. 1( a). Air is drawn into a compressor 1, which compresses the air to a pressure of between zero and one atmosphere above atmospheric pressure. The air then enters a heat exchanger 2, where it is cooled. It then moves into a chamber 3, where a valve 4 periodically opens, releasing the pressure and causing the gas to expand suddenly and adiabatically, cooling to a temperature where water droplets condense out of the air. A water-droplet collector 5 (which may be a simple filter, an inertial device such as a centrifugal separator/impactor or an electrostatic collector) then removes the droplets from the air. The water is collected in a container 6. The cold, dried air then moves into the return channel of the heat exchanger, where it cools the incoming air.

Ions are added to the compressed air to act as condensation nuclei. Water vapour will not condense from perfectly clean air unless it becomes very greatly supersaturated (relative humidities of several hundred percent), a free surface of water or ice is in contact with it (“Elementary Meteorology”, 1981, HMSO Press Edinburgh, ISBN 0 11 400312 2), or if condensation nuclei are present. Such nuclei allow droplets to grow at lower degrees of saturation than would be required without nuclei. Lower degrees of saturation are achieved at lower expansion ratios, which are clearly an advantage for energy efficient devices. In this invention nucleation sites are provided in the form of electrically generated ions, provided by a suitable high voltage process. FIG. 2 shows an example of the expansion ratio (pressure drop) required to initiate condensation for ion-containing air 7 and clean filtered air 8 at different relative humidity levels. The reduction in expansion ratio ensures that the condensation forms at the lowest possible energy cost.

The largest part of the energy needed to operate this system is used in compressing the air before it is cooled and expanded. The work required to adiabatically compress a cubic metre of air initially at atmospheric pressure P₀ to volume V is

${W_{a} = {P_{0}\left( {\frac{V^{({1 - y})}}{\gamma - 1} - 1 + V} \right)}},$

where γ is the adiabatic exponent, approximately equal to 1.4 for air. The terms (−1+V) inside the bracket are present because the compression is done in an environment at atmospheric pressure, so the pressure of air assists the compression.

From FIG. 2, it can be seen that a pressure difference of between 2 and 3 pounds per square inch (psi) is needed for condensation of water, from air at room temperature, in the presence of ions. A pressure of 3 psi corresponds to a volume change during adiabatic compression from one cubic metre to 0.88 cubic metres, and so according to the equation above, the work needed is 1173 J for every cubic metre of air processed. Using basic assumptions we can use this equation to calculate the energy cost per unit of extracted water. As an example, consider a pump working at efficiency of 0.5 and collection of 2 g of water from each cubic metre of air, we find an energy cost of ˜1,200 J/g of water collected. More detailed calculations show that such assumptions are not unrealistic—air can cool by ˜10° C. under these adiabatic expansion conditions, which can result in ˜2 g of water becoming available for condensation. However, the exact energy cost depends on inlet air temperature/humidity, heat exchanger efficiency, pump efficiency and other system losses.

FIG. 3 shows example calculations based on inlet air at 27° C. with 40% (9), 50% (10) and 60% (11) relative humidity, using a heat exchanger of 85% efficiency. The energy requirement is significantly lower than the energy used by prior art devices. For example, the EnergyStar requirement for energy efficient dehumidifiers is that they operate at less than approximately 3,000 J/g for small devices, and less than approximately 1,400 J/g for large devices. Small, commercially available devices typically operate at between 2,000 and 2,500 J/g. The method according to this invention therefore provides a lower energy method to extract water from a humid gas.

An efficient method of constructing a system which carries out the steps shown in FIG. 1( a) is shown in FIG. 1( b). A chamber 12 contains a fan 13, which is capable of working against pressures up to 0.2 atmospheres. An ion generator 14 is positioned within chamber 12. Fans driven by brushless DC motors of the type used in portable vacuum cleaners are suitable, or other types of blower can be used. The chamber has four valves 15, 16, 17 and 18 which can be opened and closed by electronic control mechanisms. The system operates in a cycle as follows (with valves closed unless otherwise indicated).

The steps in the cycle are shown in FIGS. 4( a)-4(d). At every stage, air is driven by the fan 13:

(a) Valve 18 is open. Air is drawn into the chamber 12 and compressed.

(b) Valves 16 and 17 are open. The compressed air is passed through the heat exchanger 19 and cooled.

(c) Valve 15 opens. The air expands suddenly and droplets of water condense.

(d) Valves 15 and 18 are open. The air containing water droplets is passed through a droplet collector 20 and then cools the heat exchanger 19, before being expelled from the system. At the same time, fresh air is drawn in through valve 18.

The device can also provide a sensor 21 to monitor the humidity level of the inlet air, and adjust the compression ratio accordingly. As shown in FIG. 2, in order to initiate condensation, lower compression ratios are required at higher humidity. The device ensures that the minimum required compression ratio to condense water is used to ensure maximum efficiency of operation of the device.

A different design which is also able to carry out the steps (a) to (d) described above is shown in FIG. 5. Again there is a chamber 22, with independently controlled valves 23, 24, 25 and 26, with valves 24 and 25 leading to a counter-flow air-to-air heat exchanger 27. In this design there is also a piston 28 in the chamber, which can be moved up or down by an external actuator (not shown in the diagram). Exactly the same cycle (a)-(b)-(c)-(d) described above is used in the system of FIG. 4. However, in the system of FIG. 5, air is compressed at stage (a) by a pump 29, which is connected to the chamber 22 by an additional valve 30. An ion generator 14 is positioned within chamber 22. In stages (b) and (d) it is moved through the system by motion of the piston 28 as shown in FIG. 6. Water droplets are collected in chamber 31.

Components of the system are carefully chosen in order to maximise energy efficiency.

The inlet air is filtered using an air filter (not shown) to remove airborne contaminants and particulates such as dust, dirt, mould, fungi, bacteria, pollen etc. It is constructed from a porous medium such as fibreglass, polyester, polypropylene etc and has a small pressure drop across its surfaces. The pressure drop is ideally <0.01 psi. Suitable products include the Microban HEPA filters from DuPont.

The pump is also chosen for maximum energy efficiency and should have a wall-plug efficiency of >50%, ideally >80%. It should be capable of achieving pressures of approximately 5 psi or greater. The flow rate of air provided by the pump is determined by the quantity of water that is required to be collected. As an example, to collect 500 ml of water per hour, using a device which is capable of extracting 2 g of water per cubic metre of air, a flow rate ˜>250 cubic metres per hour is required.

The heat exchanger can be of counter-flow or cross-flow type. Again it should have a minimal pressure drop across it, typical values are 0.01 to 0.05 psi. It should be capable of efficiencies greater than approximately 80%.

The ions should be provided by a suitable source, such as a high voltage ion generator. Ions should be generated in excess of the number of water droplets in the expanded gas. In the atmosphere, condensation nuclei exist in the form of Aitken nuclei (5×10⁻³ to 2×10⁻¹ microns), large nuclei (0.2 to 1 micron) and giant nuclei (>1 micron). Concentrations of these nuclei range from 10⁶/m³ over oceans to 10¹²/m³ near industrial areas. Mason (1951 Proc. Phys. Soc. B 64 773) suggested that the maximum concentration of water droplets for high expansion ratios lies in the range 1−5×10¹²/m³. Water droplets with size greater than about 5 microns are stable in a relative humidity of about 100% (Elementary Meteorology, 1981, HMSO Press). Considering a volume of air with a water content of 10 g/m³, condensed into droplets of size 5 microns, the concentration of droplets would be approximately 1.6×10⁸/m³. Based on these considerations, the concentrations of ions should be in excess of 10⁹−10¹²/m³. Ion concentrations in this range can be achieved with electrostatic ion generators.

The valves used for expansion should be capable of opening very rapidly, ideally within milliseconds, and should not restrict the rapid flow of air of the expanding gas. Solenoid, diaphragm or ball valves are suitable for this purpose provided the solenoid aperture is greater than approximately 25 mm.

The present invention provides a more efficient method to extract water from the gas. The single air flow is used in the most effective manner to pre-cool the inlet air, and in some cases, to cool the hot elements of the cooling element. The advantages of the adiabatic expansion device are associated with the lower compression/expansion ratio due to use of ions as nucleation sites. This results in a device which is more efficient than prior art devices. Furthermore, the design uses a heat exchanger to cool the compressed air with the expanded, cool air. This results in additional energy savings. Finally, the compression ratio is adjusted according to the relative humidity to further improve the energy efficiency.

Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.

INDUSTRIAL APPLICABILITY

This device has applications in a range of technologies, which may be categorized as follows:

-   -   1. Collection of clean, potable water for consumption or other         use such as cleaning, washing, etc.     -   2. Dehumidification of a particular environment such as a car,         airplane, home, or inside of an apparatus, store or container,         etc. where dry conditions are desirable.     -   3. Accumulating and maintaining or using a volume of water for         use in agriculture, domestic ‘grey water’, automobile cooling         system, non-sterile washing, cleaning applications, etc.     -   4. Drying air which has been, or will be, used to dry other         objects such as clothes, dishes, or human hair.         For categories [1] and [3], water can also be extracted from air         that is purposefully humidified by saline water. 

1. An apparatus for collecting water from air, comprising: a compressor for compressing incoming ambient air; a counter-flow heat exchanger having an inbound portion and an outbound portion, the heat exchanger receiving compressed air from the compressor in the inbound portion; an adiabatic expansion cooling device to further cool the compressed air and to condense water therefrom; an ion source for providing condensation nuclei to the adiabatic expansion cooling device; and a water collector for collecting the condensed water from the cooling device; wherein the outbound portion of the counter-flow heat exchanger is configured to receive the cooled dry air from the adiabatic expansion cooling device to pre-cool the compressed air in the inbound portion before the dry air exits the outbound portion.
 2. The apparatus of claim 1 wherein ion source comprises an electrostatic ion generator.
 3. The apparatus of claim 1 further comprising an air filter to filter the incoming ambient air.
 4. The apparatus of claim 1 further comprising a humidity sensor.
 5. The apparatus of claim 1 wherein the counter-flow heat exchanger comprises an air-to-air heat exchanger.
 6. A method for collecting water from air, comprising: flowing compressed air from a compressor to a counter-flow heat exchanger having an inbound portion and an outbound portion, the heat exchanger receiving the compressed air from the compressor in the inbound portion; cooling the compressed air in an adiabatic expansion cooling device; providing condensation nuclei to the adiabatic expansion cooling device from an ion source; condensing water from the cooled compressed air; and collecting the condensed water from the cooling device in a water collector; wherein the outbound portion of the counter-flow heat exchanger is configured to receive the cooled dry air from the adiabatic expansion cooling device to pre-cool the compressed air in the inbound portion before the dry air exits the outbound portion. 